Proteolytic enzymes play an important role in a variety of normal and abnormal physiological processes including blood coagulation, fibrinolysis, release of hormones, fertilization, and viral multiplication. Synthetic enzyme inhibitors developed for in vitro studies of specificity and mechanism are providing the basis for extended studies aimed at producing substrate-like inhibitors capable of ultimately acting selectively in vivo. The enzymes involved are chiefly serine proteinases. Two major classes of inhibitors are being evaluated. One group comprise active-site directed alkylating agents, chloromethyl ketones derived from substrates, that become attached to an active center histidine. The second group consists of acyl-analogues that hang up at the active center serine as poor substrates that cannot complete the catalytic process and result in inactive enzyme. Current interest is in exploring the selectivity possible in this approach with thrombin, plasmin, plasminogen activators, and plasma kallikrein as targets. A sulfhydryl enzyme, cathepsin B, is also under study because it is inputed in the control of protein turnover. Its response in vivo to the affinity labels under study is essential for interpretation of experiments in intact cells. Bibliographic references: Synthetic Protease Inhibitors by Affinity Labeling. E.N. Shaw. in Proteases and Biological Control, Reich, Rifkin, and Shaw, Eds., Cold Spring Harbor (1975), pp. 455-465. Progress in Designing Small Inhibitors which Discriminate Among Trypsin- like Enzymes. E.N. Shaw. In Bayer Symposium 5, Proteinase Inhibitors, Fritz, Tschesche, Greene and Truscheit, Eds., Springer-Verlag (1974), 531-540.